Merge 2024-06 CWG Motion 1

P3345R0 Core Language Working Group "ready" Issues, all issues except 2819, 2858, and 2876 (and note that 2144 is subsumed by 2876)

Author: tkoeppe

source/basic.tex

@@ -511,7 +511,9 @@
 either:
 \begin{itemize}
 \item the intervening scope is a block scope, or
-\item the intervening scope is the function parameter scope of a \grammarterm{lambda-expression}
+\item the intervening scope is the function parameter scope of a \grammarterm{lambda-expression}, or
+\item the intervening scope is the lambda scope of
+a \grammarterm{lambda-expression}
 that has a \grammarterm{simple-capture}
 naming the entity or has a \grammarterm{capture-default}, and
 the block scope of the \grammarterm{lambda-expression}
@@ -2664,28 +2666,18 @@
 
 \pnum
 \indextext{translation unit}%
-A name is said to have \defn{linkage} when it can denote the same
-object, reference, function, type, template, namespace or value as a
-name introduced by a declaration in another scope:
-\begin{itemize}
-\item When a name has \defnadj{external}{linkage},
-the entity it denotes
-can be referred to by names from scopes of other translation units or
-from other scopes of the same translation unit.
-
-\item When a name has \defnx{module linkage}{linkage!module},
-the entity it denotes
-can be referred to by names from other scopes of the same module unit\iref{module.unit} or
-from scopes of other module units of that same module.
-
-\item When a name has \defnadj{internal}{linkage},
-the entity it denotes
-can be referred to by names from other scopes in the same translation
-unit.
-
-\item When a name has \indextext{linkage!no}\defn{no linkage}, the entity it denotes
-cannot be referred to by names from other scopes.
-\end{itemize}
+A name can have
+\defnadj{external}{linkage},
+\defnadj{module}{linkage},
+\defnadj{internal}{linkage}, or
+\defnadj{no}{linkage},
+as determined by the rules below.
+\begin{note}
+All declarations of an entity with a name with internal linkage
+appear in the same translation unit.
+All declarations of an entity with module linkage
+are attached to the same module.
+\end{note}
 
 \pnum
 \indextext{linkage!\idxcode{static} and}%
@@ -2746,6 +2738,24 @@
 has its linkage determined as follows:
 \begin{itemize}
 \item
+\indextext{friend function!linkage of}%
+if the entity is a function or function template
+first declared in a friend declaration and
+that declaration is a definition and
+the enclosing class is defined within an \grammarterm{export-declaration},
+the name has the same linkage, if any,
+as the name of the enclosing class\iref{class.friend};
+\item
+otherwise,
+\indextext{friend function!linkage of}%
+if the entity is a function or function template
+declared in a friend declaration and
+a corresponding non-friend declaration is reachable,
+%FIXME: Which declaration is "that prior declaration"?
+%FIXME: "prior" with respect to what? And what about dependent lookup?
+the name has the linkage determined from that prior declaration,
+\item
+otherwise,
 if the enclosing namespace has internal linkage,
 the name has internal linkage;
 \item
@@ -3450,9 +3460,10 @@
 The \defn{lifetime} of an object or reference is a runtime property of the
 object or reference.
 A variable is said to have \defnadj{vacuous}{initialization}
-if it is default-initialized and,
+if it is default-initialized, no other initialization is performed, and,
 if it is of class type or a (possibly multidimensional) array thereof,
-that class type has a trivial default constructor.
+a trivial constructor of that class type is selected for
+the default-initialization.
 The lifetime of an object of type \tcode{T} begins when:
 \begin{itemize}
 \item storage with the proper alignment and size
@@ -4431,7 +4442,7 @@
 
 \pnum
 When an object of class type \tcode{X}
-is passed to or returned from a function,
+is passed to or returned from a potentially-evaluated function call,
 if \tcode{X} has at least one eligible copy or move constructor\iref{special},
 each such constructor is trivial,
 and the destructor of \tcode{X} is either trivial or deleted,
@@ -4454,11 +4465,6 @@
 \indextext{temporary!constructor for}%
 \indextext{temporary!destructor for}%
 \indextext{temporary!destruction of}%
-When an implementation introduces a temporary object of a class that has a
-non-trivial constructor\iref{class.default.ctor,class.copy.ctor},
-it shall ensure that a constructor is called for the temporary object.
-Similarly, the destructor shall be called for a temporary with a non-trivial
-destructor\iref{class.dtor}.
 Temporary objects are destroyed as the last step
 in evaluating
 the full-expression\iref{intro.execution}
@@ -4476,7 +4482,7 @@
 \pnum
 \indextext{initializer!temporary and declarator}%
 \indextext{temporary!order of destruction of}%
-There are four contexts in which temporaries are destroyed at a different
+There are five contexts in which temporaries are destroyed at a different
 point than the end of the full-expression.
 The first context is when a default constructor is called to initialize
 an element of an array with no corresponding initializer\iref{dcl.init}.
@@ -4602,6 +4608,14 @@
 the object persists for the lifetime of the reference
 initialized by the \grammarterm{for-range-initializer}.
 
+\pnum
+The fifth context is when a temporary object
+is created in a structured binding declaration\iref{dcl.struct.bind}.
+Any temporary objects introduced by
+the \grammarterm{initializer}{s} for the variables
+with unique names
+are destroyed at the end of the structured binding declaration.
+
 \pnum
 Let \tcode{x} and \tcode{y} each be either
 a temporary object whose lifetime is not extended, or
@@ -5760,6 +5774,15 @@
 An extended floating-point type with the same set of values as
 more than one cv-unqualified standard floating-point type
 has a rank equal to the rank of \keyword{double}.
+\begin{tailnote}
+The treatment of \tcode{std::float64_t} differs from
+that of the analoguous \tcode{_Float64} in C,
+for example on platforms where all of
+\tcode{\keyword{long} \keyword{double}},
+\keyword{double}, and
+\tcode{std::float64_t}
+have the same set of values (see \IsoCUndated{}:2024 H.4.2).
+\end{tailnote}
 \end{itemize}
 \begin{note}
 The conversion ranks of floating-point types \tcode{T1} and \tcode{T2}
@@ -5878,7 +5901,8 @@
 \item
 an immediate invocation\iref{expr.const},
 \item
-an \grammarterm{init-declarator}\iref{dcl.decl} or
+an \grammarterm{init-declarator}\iref{dcl.decl}
+(including such introduced by a structured binding\iref{dcl.struct.bind}) or
 a \grammarterm{mem-initializer}\iref{class.base.init},
 including the constituent expressions of the initializer,
 \item

source/classes.tex

@@ -1217,7 +1217,7 @@
 has a default argument
 (including the case of a constructor with no parameters).
 \indextext{implicitly-declared default constructor}%
-If there is no user-declared constructor for class
+If there is no user-declared constructor or constructor template for class
 \tcode{X},
 a non-explicit constructor having no parameters is implicitly declared
 as defaulted\iref{dcl.fct.def}.
@@ -4860,12 +4860,6 @@
 \end{codeblock}
 \end{example}
 
-\pnum
-\indextext{friend function!linkage of}%
-A function first declared in a friend declaration
-has the linkage of the namespace of which it is a member\iref{basic.link}.
-Otherwise, the function retains its previous linkage\iref{dcl.stc}.
-
 \pnum
 \indextext{declaration!overloaded name and \tcode{friend}}%
 \begin{note}

source/compatibility.tex

@@ -1997,6 +1997,28 @@
 \end{codeblock}
 \end{example}
 
+\diffref{expr.typeid}
+\change
+Evaluation of operands in \keyword{typeid}.
+\rationale
+Introduce additional expression value categories.
+\effect
+Valid \CppIII{} code that uses xvalues as operands for \keyword{typeid}
+may change behavior in this revision of \Cpp{}.
+\begin{example}
+\begin{codeblock}
+void f() {
+  struct B {
+    B() {}
+    virtual ~B() { }
+  };
+
+  struct C { B b; };
+  typeid(C().b);    // unevaluated in \CppIII{}, evaluated in \CppXI{}
+}
+\end{codeblock}
+\end{example}
+
 \diffref{expr.mul}
 \change
 Specify rounding for results of integer \tcode{/} and \tcode{\%}.
@@ -2023,6 +2045,31 @@
 \end{codeblock}
 \end{example}
 
+\diffref{expr.cond}
+\change
+Fewer copies in the conditional operator.
+\rationale
+Introduce additional expression value categories.
+\effect
+Valid \CppIII{} code that uses xvalues as operands for the conditional operator
+may change behavior in this revision of \Cpp{}.
+\begin{example}
+\begin{codeblock}
+void f() {
+  struct B {
+    B() {}
+    B(const B&) { }
+  };
+  struct D : B {};
+
+  struct BB { B b; };
+  struct DD { D d; };
+
+  true ? BB().b : DD().d;       // additional copy in \CppIII{}, no copy or move in \CppXI{}
+}
+\end{codeblock}
+\end{example}
+
 \rSec2[diff.cpp03.dcl.dcl]{\ref{dcl.dcl}: declarations}
 
 \diffref{dcl.spec}

source/declarations.tex

@@ -1512,8 +1512,10 @@
 \indextext{name!elaborated!\idxcode{enum}}%
 If an \grammarterm{elaborated-type-specifier} is the sole constituent of a
 declaration, the declaration is ill-formed unless it is an explicit
-specialization\iref{temp.expl.spec}, an explicit
-instantiation\iref{temp.explicit} or it has one of the following
+specialization\iref{temp.expl.spec},
+a partial specialization\iref{temp.spec.partial},
+an explicit
+instantiation\iref{temp.explicit}, or it has one of the following
 forms:
 
 \begin{ncsimplebnf}
@@ -1527,7 +1529,7 @@
 The second case shall appear only
 in an \grammarterm{explicit-specialization}\iref{temp.expl.spec} or
 in a \grammarterm{template-declaration}
-(where it declares a partial specialization\iref{temp.decls}).
+(where it declares a partial specialization).
 The \grammarterm{attribute-specifier-seq}, if any, appertains
 to the class or template being declared.
 
@@ -4629,15 +4631,10 @@
 If
 \tcode{T}
 is a (possibly cv-qualified) class type\iref{class}, then
-\begin{itemize}
-\item
-if \tcode{T} has
-either no default constructor\iref{class.default.ctor} or a default
-constructor that is user-provided or deleted, then the object is default-initialized;
-\item
-otherwise,
-the object is zero-initialized and then default-initialized.
-\end{itemize}
+let \tcode{C} be the constructor selected to
+default-initialize the object, if any.
+If \tcode{C} is not user-provided, the object is first zero-initialized.
+In all cases, the object is then default-initialized.
 
 \item
 If
@@ -4708,7 +4705,7 @@
 The
 \indextext{type!destination}%
 \term{destination type}
-is the type of the object or reference being initialized and the
+is the cv-unqualified type of the object or reference being initialized and the
 \term{source type}
 is the type of the initializer expression.
 If the initializer is not a single (possibly parenthesized) expression, the
@@ -4769,21 +4766,21 @@
 is sequenced before those associated with
 the initialization of the $j^\text{th}$ element.
 \item
-Otherwise, if the destination type is a (possibly cv-qualified) class type:
+Otherwise, if the destination type is a class type:
 
 \begin{itemize}
 \item
 If the initializer expression is a prvalue
 and the cv-unqualified version of the source type
-is the same class as the class of the destination,
+is the same as the destination type,
 the initializer expression is used to initialize the destination object.
 \begin{example}
 \tcode{T x = T(T(T()));} value-initializes \tcode{x}.
 \end{example}
 \item
 Otherwise, if the initialization is direct-initialization,
 or if it is copy-initialization where the cv-unqualified version of the source
-type is the same class as, or a derived class of, the class of the destination,
+type is the same as or is derived from the class of the destination type,
 constructors are considered.
 The applicable constructors
 are enumerated\iref{over.match.ctor}, and the best one is chosen
@@ -4880,7 +4877,7 @@
 the (possibly converted) value of the initializer expression.
 A standard conversion sequence\iref{conv} is used
 to convert the initializer expression to
-a prvalue of the cv-unqualified version of
+a prvalue of
 the destination type;
 no user-defined conversions are considered.
 If the conversion cannot
@@ -6263,10 +6260,9 @@
 \item from a floating-point type \tcode{T} to another floating-point type
 whose floating-point conversion rank is neither greater than nor equal to
 that of \tcode{T},
-except where the source is a constant expression and
-the actual value after conversion
-is within the range of values that can be represented (even if it cannot be represented exactly),
-or
+except where the result of the conversion is a constant expression and
+either its value is finite and the conversion did not overflow, or
+the values before and after the conversion are not finite, or
 
 \item from an integer type or unscoped enumeration type to a floating-point type, except
 where the source is a constant expression and the actual value after conversion will fit
@@ -7105,6 +7101,10 @@
 Each $\tcode{v}_i$ is the name of an lvalue of type $\tcode{T}_i$
 that refers to the object bound to $\tcode{r}_i$;
 the referenced type is $\tcode{T}_i$.
+The initialization of \exposid{e} is
+sequenced before the initialization of any $\tcode{r}_i$.
+The initialization of each $\tcode{r}_i$ is
+sequenced before the initialization of any $\tcode{r}_j$ where $i < j$.
 
 \pnum
 Otherwise,
@@ -7477,11 +7477,26 @@
 \pnum
 A \grammarterm{using-enum-declarator}
 names the set of declarations found by
-lookup\iref{basic.lookup.unqual,basic.lookup.qual}
-for the \grammarterm{using-enum-declarator}.
+type-only lookup\iref{basic.lookup.general}
+for the \grammarterm{using-enum-declarator}\iref{basic.lookup.unqual,basic.lookup.qual}.
 The \grammarterm{using-enum-declarator}
 shall designate a non-dependent type
 with a reachable \grammarterm{enum-specifier}.
+\begin{example}
+\begin{codeblock}
+enum E { x };
+void f() {
+  int E;
+  using enum E;                 // OK
+}
+using F = E;
+using enum F;                   // OK
+template<class T> using EE = T;
+void g() {
+  using enum EE<E>;             // OK
+}
+\end{codeblock}
+\end{example}
 
 \pnum
 A \grammarterm{using-enum-declaration}